1. Field of the Invention
The present invention relates generally to techniques for measuring thin insulating layers on a semiconductor wafer, and more particularly, to measurements of very thin metal oxide layers accumulated on C4 solder connections.
2. Description of Related Art
Solder connections composed of PbSn material are generally used for electrical and mechanical attachment for integrated circuit fabrication. Undesired residual metal oxides on the surface of C4 solder connections or balls can prevent an integrated circuit chip from successfully joining or wetting to a chip carrier. A thin layer of metal oxide on the surface of these solder joints will detrimentally contribute to failed electrical connections, and may compromise the chip's mechanical integrity. The metal oxides are non-conductive, having depositions that are difficult to minimize or eliminate. Oxide build-up on solder joints, such as the C4 balls, poses unique problems to the integrated circuit chip manufacturer. The ability to monitor and measure expeditiously the oxide accumulation on a substrate, particularly on the solder joints or balls of the substrate surface, within a real-time, non-destructive measurement, would greatly enhance chip yield, and decrease or eliminate infantile failures commonly associated with the fabrication process. Monitoring oxide thickness of these electrical connections would help minimize the solder joints that remain non-wettable, and facilitate inspecting the health of the manufacturing line.
Oxide thickness has been monitored previously within the art; however, the measurement techniques implemented have been limited in speed and simplicity, and at times have been shown to be destructive to the device under test.
In U.S. Pat. No. 5,485,091, issued to Verkuil on Jan. 16, 1996, entitled "CONTACTLESS ELECTRICAL THIN OXIDE MEASUREMENTS," a method using corona discharge was taught for measuring the thickness of very thin oxide layers on a silicon substrate. This method involved the active deposition of charge using a corona discharge gun, and then correlated the change in surface potential to layer thickness.
Other passive measurements have also been introduced, using optical or electrical instrumentation. For example, in U.S. Pat. No. 5,343,293 issued to Berger et al., on Aug. 30, 1994, entitled "ELLIPSOMETER," an optical ellipsometer is used to measure the thickness of oxide films on silicon wafers based on a discernible change in polarized light passing through the film.
Other oxide thickness measurements have relied upon the break down voltage of the oxide dielectric to ascertain the approximate thickness of the film deposition. This, however, is considered a destructive test since the punch-through voltage is capable of delivering additional electrical stress to the product.
Additionally, these methods are not well suited for in-situ monitoring during wafer fabrication. The equipment necessary for implementation is costly and burdensome to operate, typically requiring specially trained technicians. A less time-consuming measurement which can accurately account for extremely thin oxide films, on the order of forty (40) to one hundred sixty angstroms (160), without requiring extensive deviation to the fabrication process, remains an unresolved need in the art.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide a method for measuring oxide thickness during semiconductor fabrication utilizing general electrical parameter measurements that can be easily tested within the fabrication process.
It is another object of the present invention to provide a non-destructive oxide thickness measurement to be used in-situ during semiconductor wafer fabrication.
A further object of the invention is to provide a technique for measuring oxide layer thickness in the regime of forty to one hundred sixty angstroms on solder connections during wafer fabrication.
It is yet another object of the present invention to provide a real-time, non-destructive measurement of oxide layer thickness on a C4 solder connection of a semiconductor substrate.
Still other advantages of the invention will in part be obvious and will in part be apparent from the specification.